Shareholders' Equity - Reconciliation of Weighted Average Number of Shares (Detail) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Weighted Average Number of Shares Outstanding [Line Items]
Weighted average number of common shares outstanding	61,500	61,100	62,200
Director Plan and deferred stock units	200	200	200
ESOP	200	100	100
Common shares outstanding—basic	61,900	61,400	62,500
Common and potential common shares outstanding—diluted	62,600	62,300	63,900
Restricted Stock
Weighted Average Number of Shares Outstanding [Line Items]
Dilutive effect of stock awards	500	600	700
Stock Options
Weighted Average Number of Shares Outstanding [Line Items]
Dilutive effect of stock awards	200	300	700